The present invention concerns a precision measuring instrument for measuring dimensions of rigid objects, such as the diameter of a sphere or of a cylinder, or the side of a cube, i.e. for measuring the linear distance separating two points, two straight lines or two parallel planes of an object. Sliding calipers and micrometers are two well known examples of such instruments which in appropriate embodiments may also serve to measure inside dimensions such as the inner diameter of a tube.
Such an instrument basically comprises an elongate fixed member, a mobile member mounted for linear movement along the fixed member, each member further having a bearing element adapted to contact the object to be measured, and a measuring device, for example a vernier, that indicates the distance separating the parts of the two bearing elements that come to touch the object.
To take a measurement, firstly the bearing element of the fixed member must be brought into contact with the object to be measured, then the mobile member must be slid until its bearing element also comes into contact with the object. The contact between the bearing elements and the object must be maintained manually or by means of a locking device, and lastly the measured distance separating the contacting points of the two bearing elements must be read on the measuring device, this distance corresponding to the dimension being measured.
The invention concerns more particularly the locking device of such an instrument. In known instruments, such a locking device is, for example, made up of a locking screw which when actuated exerts a strong pressure between the fixed and mobile parts of the instrument, or of a manually-actuable lever which when released produces friction between the parts. Locking screws however have the drawback that if the instrument is not carefully manipulated and is moved away from its measuring position, the locking screw will produce large stresses on the bearing elements leading to a rapid wear thereof. The same applies to micrometers whose measuring and locking devices are made up of a micrometric screw that is liable to produce substantial pressures on the object to be measured. Friction-producing levers have the drawback that a precise measurement cannot be guaranteed because it is always possible for one of its parts to move relative to the other. These drawbacks become all the more serious as the difficulty of placing the instrument on the object in the best position for giving an exact measurement increases, for example for taking an inside measurement.